Will it be correct to delta hedge one SPX put with 2 ES contracts? any other viable/liquid possibility (besides the too expensive 100 SPY shares)? Tnx

That depends on the put's delta. ES has a delta of 50 (i.e. each point is worth $50, while each point in SPX is worth 100), so 2 ES means a delta of 100.

IOW, to hedge 40 SPX Feb ATM calls , I need to short 40 ES contracts. Reg T margin for this is $320,000. Anybody knows if PM would be lighter for this position?

Tnx again. sorry to go on about this, but it is important and practical for me. How about hedging with SP futures? e.g. long one future contract for every 5 SPX ATM puts? will that work (delta-wise)? would that reduce the margin requirement ('cause option and future are for the same U/L)?

You're replicating the ATM straddle, synthetically. You would need ~10 ES against your puts. Why not simply go long or short the natural straddle in ES (or even the synthetic) and receive SPAN treatment?